Method of making galvannealed ferrous metal of improved solderability



United States Patent Office 3,304,245 Patented F eb. 14, 1967 METHOD OF MAKING GALVANNEALED FER- ROUS METAL F IMPROVED SOLDERA- BILITY Lawrence E. Heiwig, Glenshaw, Pa., assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Aug. 20, 1964, Ser. No. 390,993 3 Claims. (Cl. 204-35) The present invention relates to a method of making galvannealed ferrous metal of improved solderability. The terms ferrous and ferrous metal as used herein refers to iron and iron alloys.

Galvannealed ferrous metal articles such as sheet, strip, etc., are produced by first galvanizing a ferrous metal base to provide a coating of zinc. The normal zinc metal used in galvanizing, i.e. the spelter, contains lead, iron, and aluminum. In galvannealing, the galvanized surface is heated so that the zinc coating substantially completely alloys with the ferrous metal base. The galvannealed surface has a silvery matte finish of low reflectivity.

Although galvanized ferrous metal products are readily solderable, galvannealed material is normally regarded as unsolderable. I have discovered that the poor solderability of galvannealed ferrous metal is due to the presence at the coating surface of an oxide film which includes aluminum oxide. This oxide film is an effective insulating barrier that is particularly impenetrable to the reducing action of fluxes normally used in soldering operations.

It is an object of my invention to produce galvannealed ferrous metal of improved solderability by partially or totally removing the undesirable surface oxide film. According to my invention, a ferrous metal base is galvanized by providing an aluminum-containing zinc coating onto a ferrous metal base and thereafter galvannealing the galvanized ferrous metal by heating to substantially completely alloy the coating with the base. The coating surface is then cathodically treated with at least 1000 coulombs/ft. of electricity. The cathodic treatment changes the character of the galvannealed surface in such a way as to render it solderable.

Although it would appear that the problem of solderability of galvannealed ferrous metal articles could be the micrometer and serves as a useful measure of the solderability of the material being evaluated.

The following examples illustrate various embodiments which can be used. Samples of galvannealed sheet having a coating of zinc alloy (0.76 oz./ft. sheet) containing 0.17% aluminum, 0.16% lead, was cathodically, reduced in an electrolyte solution of aqueous sodium carbonate. An electrolytic cell with stainless steel anodes was used in which 3- by 6-inch panel samples of the galvannealed sheet were electrically connected as cathodes. A variation of surface conditions was achieved by holding the current density constant at 60 amperes/ft. of surface area and varying the reduction time. The cathodically treated samples were evaluated for solder-performance and it was concluded that satisfactory solderability could be obtained after the surface oxide of the galvannealed samples had been cathodically treated with at least 1000 coulombs per square foot of surface. The effectiveness of the cathodic treatment increases with coulombic inputs up to about 3000 coulombs at which point maximum effectiveness is obtained. Current densities of at least about 20 amps/ft. are generally necessary to provide the quantity of electricity (coulombs) required to accomplish the cathodic treatment necessary for good solderability in a reasonably short time period.

Extensive testing has shown that satisfactory improvement in solderability can ony be achieved when a sufficient coulombic or electrochemical treatment of the coated surface is achieved. A coulomb is a measure of a quantity of electricity and by international agreement one coulomb is the amount of electricity which deposits 0.001118 gram of silver. An electric current carrying one coulomb per second is referred to as a current of one ampere.

The effect on solderability caused by the cathodic treatment is illustrated by the following examples. A series of tests were conducted using different concentrations of electrolyte on 4" x 6" samples of the same galvannealed sheet described above. By varying current density and treating time it was possible to evaluate the variation in the solderability improvement with coulombic input. The results of these tests are given in Table I.

TABLE I Solder-DropMaximum Height After the Indicated Corresponding Coulombs Applied Electrolyte Sodium Carbonate Current Length of Cleaning Time, mils* Concentration, Percent Density amp/ft.

10 sec. 20 see. 40 see. 60 sec. 90 sec 10 see. 20 sec 40 sec. 60 sec 90 sec Low solder drop heights indicate better solder flow. I avoided by simply employing an aluminum-free zinc coating, such a practice is commercially impracticable. Aluminum is a useful addition in galvanized coatings normally manufactured on the same production lines used in producing galvannealed products. Consequently, it is not feasible to employ zinc coating materials free of aluminum.

solderability can be evaluated by a standard solderflow test in which a panel of the sample is coated with a mild flux, such as Kester No. 415, and a bead of solder placed on the fluxed surface. The panel is then heated, usually on a hotplate, to soldering temperature, e.g. 525 F., whereby the solder melts and flows on the panel surface. On a material having good solderability, the solder pellet will flow readily and attain a maximum spread and minimum thickness. Thus the better the solderability, the thinner the layer of solder flow on the panel sample. The thickness of the soldered layer can be measured with Generally, solder spot thicknesses of 45 mils and under are regarded as characteristic of a material having good solderability. At least 1000 coulombs per square foot have been found to be required to produce galvannealed surfaces within the good solderability range. Thus, for example, at a current density of 19 amps/ft. a solder drop height of 41 mils (indicating good solderability) was achieved with a 60-second treating time and a total coulombic input of 1160 coulombs. Higher current densities allow the required quantity of electricity to be supplied in a shorter time.

Comparative tests with galvannealed sheet samples whose zinc coatings contained various quantities of aluminum indicate that even small amounts of aluminum in the coating greatly reduce the solderability of the galvannealed surface. Cathodic treatment of the galvannealed surfaces greatly improves solderability over the entire range of aluminum content of the coatings which were investigated. Table II summarizes the results of a series of tests conducted with galvannealed sheet having variable aluminum content in the coating. The samples treated were 4 x 6" panels that had been galvannealed in air for 15 minutes at 1000 F.

From the foregoing description it can be appreciated that various changes and modifications may be made without departing from my invention. For example, the composition of the electrolyte is not critical and any solution can be used which provides a current flow that produces the required cathodic treatment of the surface in the desired unit time. The electrolyte should be compatible with the zinc-iron alloy coating under cathodic conditions, i.e. the electrolyte should not attack the coating under conditions of electrolysis. The concentration of the electrolytes are also not critical and are determined primarily by their current carrying capacity. The phenomenon of cathodic reduction is well known and various electrolytic cell designs for industrial application may be used. Similarly, different anode materials can be employed within the practice of my invention. The cathodic treatment technique can be used in commercial galvannealing lines without material alteration of the equipment. Rectifier or power generators and grids can be used with an electrolyte which allows 1000 coulombs of current flow in conveniently short time periods, e.g. one or two seconds. Thus, for example, for a 48" sheet at a feet per minute line speed, a power rectifier capable of producing 7000 amps would be adequate. At higher speeds more power would be required, for example 15,000 amps at 150 feet per minute. Commercially practical combinations of electrolyte concentration and power output will be governed by the desired line speed. Thus, a 10% sodium carbonate solution using a power source capable of producing 15,000 amps and a line speed of 150 feet per minute could be employed. The invention could be used on a continuous galvannealing line or as a separate procedure after galvannealing.

I claim:

1. A method of making galvannealing ferrous metal of improved solderability comprising galvanizing a ferrous metal base with an aluminum-containing spelter to provide a coating of zinc thereon, galvannealing said galvanized ferrous metal base by heating to substantially completely alloy said coating with said base and cathodically treating said coating surface in an aqueous electrolyte of the character which will not attack said coating by passing at least 1000 coulombs/ft. of electricity through said surface whereby galvannealed ferrous metal of improved 'solderability is produced.

2. A method according to claim 1 wherein a current density of at least about 20 amps/ft. is employed.

3. A method according to claim 1 wherein about 3000 coulombs/ft. is used.

References Cited by the Examiner UNITED STATES PATENTS 7/1947 Stevenson 204-1405 8/1952 Neish 204 

1. A METHOD OF MAKING GALVANNEALING FERROUS METAL OF IMPROVED SOLDERABILITY COMPRISING GALVANIZING A FERROUS METAL BASE WITH AN ALUMINUM-CONTAINING SPELTER TO PROVIDE A COATING OF ZINC THEREON, GALVANNEALING SAID GALVANIZED FERROUS METAL BASE BY HEATING TO SUBSTANTIALLY COMPLETELY ALLOY SAID COATING WITH SAID BASE AND CATHODICALLY TREATING SAID COATING SURFACE IN AN AQUEOUS ELECTROLYTE OF THE CHARACTER WHICH WILL NOT ATTACK SAID COATING BY PASSING AT LEAST 100 COULOMBS/FT.2 OF ELECTRICITY THROUGH SAID SURFACE WHEREBY GALVANNEALED FERROUS METAL OF IMPROVED SOLDERABILITY IS PRODUCED. 